<<

48525

Paper number 118 ENVIRONMENT DEPARTMENT PAPERS Change Series Environment Department THE BANK

1818 H Street, NW Washington, D.C. 20433 Telephone: 202-473-3641 Facsimile: 202-477-0565 and Level Rise A Review of the Scientific Evidence

Susmita Dasgupta and Craig Meisner

May 2009

Sustainable Development Vice Presidency Printed on recycled paper stock, using soy inks. The Environment Department

Climate Change and Rise A Review of the Scientific Evidence

Susmita Dasgupta Craig Meisner

May 2009

Financial support for this study was provided by the Research Department of the World Bank and the Canadian Trust Fund (TF030569) sponsored by the Canadian International Development Agency (CIDA). We would like to extend our special thanks to Kiran Pandey for his valuable help. We are also grateful to David wheeler, Michael Toman and Rawleston Moore for useful comments and suggestions.

Papers in this series are not formal publications of the World Bank. They are circulated to encourage thought and discussion. The use and citation of this paper should take this into account. The views expressed are those of the authors and should not be attributed to the World Bank. Copies are available from the Environment Department of the World Bank by calling 202-473-3641. © The International Bank for Reconstruction and Development/THE WORLD BANK 1818 H Street, N.W. Washington, D.C. 20433, U.S.A.

Manufactured in the United States of America May 2009

The findings, interpretation, and conclusions presented in this report are those of the author and should not be attributed to the World Bank, its affiliated organizations, or to members of its Board of Executive Directors or the countries they represent. The presentation of material in this document and the geographical designations employed do not imply expression of any opinion on the part of the World Bank concerning the legal status of a country, territory or area, or concerning the delimitation of its boundaries.

Design: Jim Cantrell Cover photos: Perito Moreno , Argentina; © Shutterstock Images, LLC

Contents

Abstract v

Section 1: Introduction 1

Section 2: Climate Change and SLR — Observations from the Past 3 thermal expansion 3 Changes in and icecaps 3 Glacial melt from and 3 on land 4 4

Section 3: — Predictions 6 Ocean thermal expansion 6 Glaciers and icecaps, Greenland , , snow on land, and permafrost 6 Projections of mass balance sensitivity of glaciers and ice caps 7 Greenland and Antarctica Ice Sheets 8 Likelihood of abrupt changes 8 Projections of global SLR by IPCC AR4 9

Section 4: Recent Evidence for the Greenland and Antaractic Ice Sheets 10 10 Antarctic Ice Sheet 11

Section 5: Other Projections of SLR 13

Section 6: Conclusion 15 Impacts of SLR on height 15

References 17

Appendix 1 — Source of Data on Major Contributory Factors to SLR 22 Ocean thermal expansion 22 Greenland Ice Sheet, Antarctic Ice Sheet 22 Glaciers and ice caps (not immediately adjacent to Greenland and Antarctic ice sheets) 23

Climate Change Series iii Climate Change and Sea Level Rise — A Review of the Scientific Evidence

Snow on Land 23 Permafrost 24

Appendix 2 — The Emission Scenarios of the IPCC Special Report 25

Endnotes 26

Tables

Table 1. Contributions to SLR Based Upon Observations as Compared to Models Used in the IPCC AR4 4 Table 2. Area, Volume and Sea Level Equivalent (SLE) of the Other Major Contributory Factors to SLR 7 Table 3. Mass Balance (MB) of the East Antarctic (EAIS), West Antarctic (WAIS), Antarctic (AIS), and Greenland (GIS) Ice Sheets as Determined by a Range of Techniques and Studies 10

iv Environment Department Papers

Abstract

ea-level rise (SLR) due to climate change is a There appears to be a consensus across studies that serious global threat: The scientific evidence is now global sea level is projected to rise during the 21st overwhelming. The rate of global sea level rise was century at a greater rate than during the period 1961 faster from 1993 to 2003, about 3.1 mm per year, to 2003 and unanimous agreement that SLR will not Sas compared to the average rate of 1.8 mm per year be geographically uniform. Ocean thermal expansion from 1961 to 2003 (IPCC, 2007); and significantly is projected to contribute significantly, and land ice will higher than the average rate of 0.1 to 0.2 mm/yr increasingly lose mass at an accelerated rate. But most increase recorded by geological data over the last 3,000 controversial are the mass balance loss estimates of the years. Anthropogenic warming and SLR will continue Greenland and Antarctic Ice Sheets and what the yet for centuries due to the time scales associated with un-quantified dynamic processes will imply in terms of climate processes and , even if SLR. Recent evidence on the of Greenland concentrations were to be stabilized. This paper reviews and west Antarctic ice sheets to climate warming raises the scientific literature to date on climate change and the alarming possibility of SLR by one meter or more sea level rise. by the end of the .

Climate Change Series v

1 Introduction

ea-level rise (SLR) due to climate change is a in coastal regions. The biophysical effects of SLR on serious global threat: The scientific evidence is now coastal regions include: i) inundation, and storm overwhelming. The rate of global sea level rise was damage; ii) loss; iii) ; iv) saltwater faster from 1993 to 2003, about 3.1 [2.4 to 3.8] intrusion v) (from higher sea water Smm per year, as compared to the average rate of 1.8 temperatures); vi) ocean productivity changes; and [1.3 to 2.3] mm per year from 1961 to 2003 (IPCC, vii) species migration. Most of these impacts are 2007); and significantly higher than the average rate of broadly linear functions of sea level rise, although 0.1 to 0.2 mm/yr increase recorded by geological data some processes such as wetland loss and change show a over the last 3,000 years.1 threshold response and are more strongly related to the rate of sea level rise, rather than the absolute change. Even if greenhouse gas (GHG) emissions were These natural-system effects have a range of potential stabilized in the near future, sea levels would continue socio-economic impacts, including the following to rise for many decades (IPCC, 2007). Until recently, identified by McLean and Tsyban (2001): i) increased studies of SLR typically predicted ranges within the loss of property and coastal , ii) increased flood 0-1 meter during the 21st century (Church et al., risk and potential loss of life, iii) damage to coastal 2001). The IPCC AR4 report has projected a rise in sea protection works and other infrastructure, iv) loss of level between 0.18 to 0.59 meters by the end of 21st renewable and subsistence resources, and v) loss of century - across different emission scenarios. However, tourism, recreation, and transportation functions. the models considered by the IPCC, 2007 did not include the full effect of changes in ice sheet flow, The impacts of SLR could be catastrophic for many and IPCC projections have been criticized by many developing countries. Recent World Bank estimates experts as being too conservative (Oppenheimer et al., suggest that even a one-meter rise in sea level in coastal 2008; Pfeffer et al., 2008; Solomon et al., 2008). New countries of the developing world would submerge measurement information on the rates of 194,000 square kilometers of land area, and displace in Greenland and the West Antarctic suggest an at least 56 million people (Dasgupta et al. 2007). The alarming possibility that a threshold triggering many actual impacts, however, will depend on countries’ meters of sea level rise could be crossed well before ability to adapt to the potential impacts of SLR. the end of this century (Hansen et al., 2007; Helsen et al., 2008; Joughin et al., 2008; Kohler et al., 2007; The impacts of climate change on sea level rise are Overpeck et al., 2006; Rignot, 2008; Van de Wall et al., complex. At play are the interactions between natural 2008).2 cycles and activity, and a full assessment of the range of climate change consequences and probabilities SLR is a serious threat to countries with high involves a cascade of in emissions, carbon concentrations of population and economic activity cycle response, climate response, and impacts. In this

Climate Change Series 1 Climate Change and Sea Level Rise — A Review of the Scientific Evidence

paper we review the scientific literature on climate future. Section 4 then provides an overview of the change and SLR to date. latest research on SLR and serves as an update to the IPCC AR4 report. Section 5 provides an overview of In the next section we begin with a summary of the some of the criticisms of the IPCC AR4 report and historical observations as noted by the IPCC in their section 6 concludes the review. latest report and in section 3, their predictions for the

2 Environment Department Papers

Climate Change and SLR — 2 Observations from the Past (as summarized in the IPCC AR4 report)

he IPCC AR4 Report identifies several major 2003 had much higher rates of warming, especially in factors that currently contribute to sea level the upper 700 m of the global ocean. change. These are: T Changes in glaciers and icecaps • Ocean thermal expansion3 During the , glaciers and ice caps have • Changes in glaciers and icecaps experienced widespread mass losses. These losses • Glacial melt from the Greenland and Antarctica (excluding those around the ice sheets of Greenland Ice Sheets and Antarctica) are estimated to have contributed 0.50 • A smaller contribution from snow on land ± 0.18 mm/yr in sea level equivalent (SLE) between 4 and permafrost (changes in other Cryospheric 1961 and 2003, and 0.77 ± 0.22 mm/yr between 1991 components which have marginal impacts). and 2003.

Below we provide a brief description of the relative contribution of these factors to SLR as observed Glacial melt from Greenland and Antarctica and modeled in the past. Table 1 summarizes these Whether the Greenland and Antarctic ice sheets contributions and Appendix I provides a more detailed have been growing or shrinking over time scales of discussion of the sources of data and procedures longer than a decade is not well established from commonly used to estimate these contributions to SLR. observations. Lack of agreement between techniques and the small number of estimates preclude assignment Ocean thermal expansion of best estimates or statistically rigorous error bounds for changes in ice sheet mass balances. However, Instrumental records reveal that the world’s have acceleration of outlet glaciers draining from the interior warmed since 1955, accounting over this period for has been observed in both the Greenland and Antarctic more than 80% of the changes in the energy content ice sheets. of the ’s . Records further reveal during the period 1961 to 2003, the 0 to 3000 m ocean According to the IPCC AR4, it is very likely (> 90% layer has absorbed up to 14.1 × 1022 Joules, equivalent probability) that the Greenland Ice Sheet (GIS) shrunk to an average heating rate of 0.2 Watts/m2 (per unit from 1993 to 2003, with the thickening in central area of the Earth’s surface). During 1993 to 2003, the regions being more than offset by increased melting corresponding rate of warming in the shallower 0 to in coastal regions. An assessment of the data suggests 700 m ocean layer was higher, about 0.5 ± 0.18 W/m2. a mass balance for the Greenland Ice Sheet of –50 to Hence, relative to 1961 to 2003, the period 1993 to –100 Gigatons/year (a shrinkage contributing to rising

Climate Change Series 3 Climate Change and Sea Level Rise — A Review of the Scientific Evidence

global sea levels of 0.14 to 0.28 mm/yr) from 1993 to decreases or no changes in the past 40 years or more. 2003, with even larger losses in 2005. The estimated Decreases in the snow pack have also been documented range in mass balance for the GIS from 1961 to 2003 is in several regions worldwide based upon annual time between a growth of 25 Gt/yr and shrinkage of 60 Gt/ series of mountain snow water equivalent and snow yr (or –0.07 to +0.17 mm/yr SLE).5 depth.

There are even greater uncertainties for Antarctica Ice Sheet (AIS). Again according to the IPCC AR4 Permafrost assessment of all the data yields an estimate for the Permafrost and seasonally frozen ground in most overall AIS mass balance ranging from growth of 100 regions display large changes in recent decades. Gt/yr to shrinkage of 200 Gt/yr (or –0.27 to +0.56 Temperature increases at the top of the permafrost mm/yr of SLE) from 1961 to 2003, and from +50 to layer of up to 3°C since the have been reported. –200 Gt/yr (or –0.14 to +0.55 mm/yr of SLE) from Permafrost warming has also been observed with 1993 to 2003. variable magnitudes in the Canadian , , the and . The permafrost base Snow on land has been thawing at a rate ranging from 0.04 m/yr in Alaska to 0.02 m/yr on the Tibetan Plateau. Snow cover has decreased in most regions, especially in spring. observations of the Northern Summarizing each of these contributors in Table 1, the Hemisphere snow cover from 1966 to 2005 show average rate of global mean SLR from 1961 to 2003, a decrease in every month except in November and estimated from gauge data, is 1.8 ± 0.5 mm/yr. December, with a stepwise drop of 5% in the annual Thermal expansion’s contribution to SLR over this mean in the late 1980s. In the Southern Hemisphere, period was 0.42 ± 0.12 mm/yr (about one-quarter the few long records or proxies mostly show either of the total observed SLR), with significant decadal

Table 1. Contributions to SLR Based Upon Observations as Compared to Models Used in the IPCC AR46 Sea Level Rise (mm/year) 1961-2003 1993-2003 Sources of SLR Observed Modeled Observed Modeled Thermal expansion 0.42 ± 0.12 0.5 ± 0.2 1.6 ± 0.5 1.5 ± 0.7 Glaciers and ice caps 0.50 ± 0.18 0.5 ± 0.2 0.77 ± 0.22 0.5 ± 0.3 Greenland Ice Sheet 0.05 ± 0.12a 0.21± 0.07a Antarctic Ice Sheet 0.14 ± 0.41a 0.21± 0.35a Sum of individual climate 1.1 ± 0.5 1.2 ± 0.5 2.8 ± 0.7 2.6 ± 0.8 contributions to SLR 3.1 ± 0.7 1.8 ± 0.5 Observed total SLR (satellite (tide gauges) altimeter) Difference (observed total minus the sum 0.7 ± 0.7 0.3 ± 1.0 of observed climate contributions)

Note: a – prescribed based upon observations. Source: Table TS.3 (IPCC 2007).

4 Environment Department Papers Climate Change and SLR — Observations from the Past

variation, while the contribution from glaciers, ice caps global average rate of sea level rise from 1993 to 2003 is and ice sheets was estimated to have been 0.7 ± 0.5 3.1 ± 0.7 mm/yr, which is close to the estimated total of mm/yr (about less than half of the total). The sum of 2.8 ± 0.7 mm/yr for the climate-related contributions these estimated contributions is about 1.1 ± 0.5 mm/yr, due to thermal expansion (1.6 ± 0.5 mm/yr) and which is less than the best estimate from the tide gauge changes in land ice (1.2 ± 0.4 mm/yr). On average, observations (see Table 1).7, 8 thermal expansion and melting of ice each accounted for about half of the observed SLR during this period, 9 More recent evidence using satellite altimetry reveals a although there is some in the estimates. much higher response of SLR to climate change. The

Climate Change Series 5

3 Sea Level Rise — Predictions (as summarized in IPCC AR4 report)

lobal sea level is projected to rise during the Ocean thermal expansion 21st century at a greater rate than during 1961 Expected global average thermal expansion can be to 2003. As in the past, there is unanimous calculated directly from simulated changes in ocean agreement that sea level change in the future will G temperature. Results are available from 17 - not be geographically uniform. Ocean General Circulation Models (AOGCMs) for However, quantitative projections of SLR have been the 21st century for SRES scenarios A1B, A2 and B1, a source of immense debate in the recent years, given continuing from simulations of the 20th century (see the number of natural processes involved and the Appendix 2 for an explanation of the different SRES scenarios). One ensemble member was used for each associated uncertainty of each systems reaction. The model and scenario.10 most important uncertainty relates to whether the discharge of ice from ice sheets will continue to increase During the period 2000 to 2020, under scenario SRES as a consequence of accelerated ice flow, as observed A1B in the ensemble of AOGCMs, the rate of thermal in recent years. Limited understanding of ice flow expansion is 1.3 ± 0.7 mm/yr, and is not significantly dynamics prevents quantitative projections of discharge different under A2 or B1. This rate is more than twice from ice sheets, and hence SLR with confidence. the observationally derived rate of 0.42 ± 0.12 mm /yr Despite these uncertainties, the IPCC does make the during 1961 to 2003. It is similar to the rate of 1.6 ± following overall observations about future SLR: 0.5 mm/yr during 1993 to 2003.11

• Climate models are consistent with the ocean During the period 2080 to 2100, the rate of thermal temperature observations and indicate that thermal expansion is projected to be 1.9 ± 1.0, 2.9 ± 1.4 and expansion is expected to continue to contribute to 3.8 ± 1.3 mm/yr under scenarios B1, A1B and A2 sea level rise over the next 100 years. Since deep respectively in the AOGCM ensemble (the width of ocean temperatures change only slowly, thermal the range is affected by the different numbers of models expansion would continue for many centuries even under each scenario). The acceleration is caused by the if atmospheric concentrations of greenhouse gases increased climatic warming. were stabilized; • The retreat of glaciers and ice caps is expected to Glaciers and icecaps, Greenland ice sheet, continue during the next 100 years and their con- Antarctic ice sheet, snow on land, and tribution should decrease in subsequent centuries permafrost as this store of freshwater diminishes; and • Although there is uncertainty about Antarctic Table 2 summarizes the current stock of ice, along with Ice Sheet, melting of the Greenland Ice Sheet is what this ice would represent in terms of sea level rise projected to increase further in the recent future. were it to completely melt.

6 Environment Department Papers Sea Level Rise — Predictions

Table 2. Area, Volume, and Sea Level Equivalent (SLE) of the other Major Contributory Factors to SLR (Indicated are the annual minimum and maximum for snow, and the annual mean for the other components. The values for glaciers and ice caps denote the smallest and largest estimates excluding glaciers and ice caps surrounding Greenland and Antarctica) Area Ice volume Potential sea level rise (106 km2) (106 km3) (m) f Glaciers and Ice caps Smallest estimate a 0.51 0.05 0.15 Largest estimate b 0.54 0.13 0.37 Ice sheets 14.0 27.6 63.9 Greenland c 1.7 2.9 7.3 Antarctica d 12.3 24.7 56.6 Snow on Land (NH*) 1.9-45.2 0.0005-0.005 0.001-0.01 Permafrost (NH) e 22.8 0.011-0.037 0.03-0.10

Notes: * Northern Hemisphere a Ohmura (2004); glaciers and ice caps surrounding Greenland and Antarctica are excluded. b Dyurgerov and Meier (2005); glaciers and ice caps surrounding Greenland and Antarctica are excluded. c Bamber et al. (2001). d Lythe et al. (2001). e Zhang et al. (1999), excluding permafrost under ocean, ice sheets and glaciers. f Assuming an oceanic area of 3.62 × 108 km2, an ice density of 917 kg m–3, a density of 1,028 kg m–3, and seawater replacing grounded ice below sea level.

Source: Table 4.1, IPCC 2007.

Table 2 highlights the potential importance of ice • For a seasonally uniform temperature rise, alterna- sheets, glaciers and ice caps for the SLR in future. The tive studies have estimated that an increase of present Antarctic and Greenland and ice sheets contain of 20 to 50% / °C will be required to enough water to raise sea level by almost 57m, and balance ablation12 increases for a sample of glaciers 7m respectively; and melting of all glaciers and icecaps and ice caps (Oerlemans et al., 1998; Braithwaite would raise sea level by 0.05m – 0.13m. et al., 2003; Would and Hock, 2006; Oerlemans et al. 2006). Although AOGCMs generally project larger than average precipitation changes in the Projections of mass balance sensitivity of northern mid- and high-latitude regions, the global glaciers and icecaps average is 1 to 2% / °C, so increases would The acceleration of glacier loss over the next few be expected to dominate worldwide. decades is likely. • The mean specific surface mass balance of a glacier or ice cap will change as volume is lost.13 Omission • Using monthly temperature changes simulated of this effect leads to overestimates of ablation. in glaciers and ice cap regions by 17 AOGCMs In order to correct projections, a scaling relation for scenarios A1B, A2 and B1, the global total between the area of a glacier and its volume was surface mass balance sensitivity to global average derived. Application of the scaling reduces the temperature change for all glaciers and ice caps projections of the glaciers and ice caps contribu- outside Greenland and Antarctica range between tion up to the mid-21st century by 25% and over 0.61 ± 0.12 mm/yr/°C (SLE) and 0.49 ± 0.13 the whole century by 40 to 50% with respect to mm/yr/°C for various modeling techniques, subject fixed glacier and ice cap areas. to uncertainty in these areas (Zuo and Oerlemans, • Estimates suggest that in future decades Antarctic 1997; Oerlemans, 2001; and Oerlemans et al. and Greenland glaciers and ice caps (not the ice 2006). sheets) will together contribute 10 to 20% to sea

Climate Change Series 7 Climate Change and Sea Level Rise — A Review of the Scientific Evidence

level as part of the contribution of other glaciers negatively to sea level, owing to increasing accumu- and ice caps. lation exceeding any ablation increase. Hence, melt water from glaciers must be considered • For an average temperature change of 3°C over as an important contributor to the total sea level rise each ice sheet, a combination of four high-resolu- expected in this century.14 tion AGCM simulations and 18 AR4 AOGCMs (Huybrechts et al., 2004; Gregory and Huybrechts, 2006) give surface mass balance changes of 0.3 ± Greenland and Antarctica Ice Sheets 0.3 mm /yr for Greenland and –0.9 ± 0.5 mm/yr The average annual solid precipitation falling onto these for Antarctica (SLE), that is, sensitivities of 0.11 ± ice sheets is equivalent to 6.5 mm of sea level this input 0.09 mm/yr/°C for Greenland and –0.29 ± 0.18 being approximately balanced by loss from melting mm/yr/°C for Antarctica.15 and calving (creation of icebergs) (IPCC, 2001). The balance of these processes is not the same for the Likelihood of abrupt changes two ice sheets, on account of their different climatic regimes. Antarctic temperatures are so low there is • Satellite and in situ measurement networks have virtually no surface runoff; where the only reduction demonstrated increasing melting and accelerated in size arises from ice discharge into the ocean and the ice flow around the periphery of the Greenland Ice formation of icebergs. Greenland, on the other hand, Sheet over the past 25 years. The few simulations experiences summertime temperatures high enough to of long-term ice sheet simulations suggest that cause widespread melting (even in the interior), which the Greenland Ice Sheet will significantly decrease accounts for half of the ice loss, with the remainder in volume and area over the coming centuries if being the discharge of ice as icebergs or into small ice- a warmer climate is maintained (Gregory et al., shelves. 2004; Huybrechts et al., 2004; Ridley et al., 2005). A threshold of annual mean warming of 1.9°C • In projections of surface mass balance changes to 4.6°C in Greenland has been estimated for the for Greenland, ablation increase is important but elimination of the Greenland Ice Sheet (Gregory uncertain, being particularly sensitive to tempera- and Huybrechts, 2006). ture change around the margins. Climate models • Recent observations of accelerated ice streams in project less warming in these low- regions the sector of the WAIS, the rapidity than the Greenland average, and less warming in of propagation of this signal upstream and the summer (when ablation occurs) than the annual acceleration of glaciers that fed the Larsen B Ice average, but greater warming in Greenland than Shelf after its collapse, have raised the concern the global average (Church et al., 2001; Huy- of a collapse of the WAIS. It is that the brechts et al., 2004; Chylek and Lohmann, 2005; presence of ice shelves tend to stabilize the ice Gregory and Huybrechts, 2006). In most studies, sheet, at least regionally. Therefore, a weakening Greenland surface mass balance changes represent or collapse of ice shelves, caused by melting on the a net positive contribution to sea level in the 21st surface or by melting at the bottom by a warmer century because the ablation increase is larger than ocean, might contribute to a potential destabiliza- the precipitation increase. tion of the WAIS, which could proceed through • On the contrary, all studies for the 21st century the positive of the grounding-line retreat. considered by the IPCC AR4 project that Antarc- However, the present understanding is insufficient tic surface mass balance changes will contribute

8 Environment Department Papers Sea Level Rise — Predictions

for prediction of the possible speed or extent of to thermal expansion, melting of glaciers-with the such a collapse. Greenland and Antarctic ice sheets calculated as being close to mass balance; and exclude rapid dynamical Projections of global SLR by IPCC AR4 changes in ice flow. According to the IPCC AR4, The IPCC AR4 report projected increased global SLR present scientific understanding is insufficient for between 0.18m and 0.59m across various emission prediction of any rapid dynamical / abrupt changes in scenarios over the next 100 years.16 These projections, ice flow. In the next section provide an overview of the however, are calculated from projections of SLR due latest evidence alongside the IPCC AR4 conclusions.

Climate Change Series 9

Recent Evidence for the Greenland 4 and Antarctic Ice Sheets

ince melting of polar ice sheets is the largest Greenland Ice Sheet determinant of future SLR, and the present As noted earlier, Greenland’s seasonal temperature Greenland and Antarctic ice sheets contain variation causes widespread melting in the interior and enough water to raise sea level by almost 70 m through the discharge of icebergs and small ice-shelves. S(Table 2), even small changes in their volume would Tedesco’s (2007) long-term results show that snowmelt have a significant effect. extent has been increasing at a rate of ~40,000 km2 Table 3 below summarizes some of the mass balance per year for the past 14 years. In addition, studies have estimates for each of the major ice sheets, several of found major short-term variations in ice discharge which were also used in the IPCC AR4 assessment. and mass loss from Greenland’s major outlet glaciers.

Table 3. Mass balance (MB) of the East Antarctic (EAIS), West Antarctic (WAIS), Antarctic (AIS), and Greenland (GIS) Ice Sheets as Determined by a Range of Techniques and Studies (Not all studies surveyed all of the ice sheets, and the surveys were conducted over different periods within the time frame 1992 to 2006. For comparison, 360 Gt of ice is equivalent to 1 mm of eustatic sea level rise) Survey Survey area EAIS MB WAIS MB AIS MB GIS MB Study period 106 km2 (%) Gt/year Gt/year Gt/year Gt/year Wingham et al., 1998 * 1992–1996 7.6 (54) -1 ± 53 - 59 ± 50 - 60 ± 76 + Krabill et al., 2000 * 1993–1999 1.7 (12) - 47 + Rignot and Thomas, 2002 † 1995–2000 7.2 (51) 22 ± 23 -48 ± 14 -26 ± 37 Davies and Li, 2004 * 1992–2002 8.5 (60) 42 ± 23 + Davies et al., 2005 * 1992–2003 7.1 (50) 45 ± 7 + Velicogna and Wahr, 2005 ‡ 2002–2004 1.7 (12) -75 ± 21 + Zwally et al., 2005 * 1992–2002 11.1 (77) 16 ± 11 -47 ± 4 -31 ± 12 11 ± 3 1996 -83 ± 28 + Rignot and Kanagaratnam, 2006 † 2000 1.2 (9) -127 ± 28 2005 -205 ± 38 Velicogna and Wahr, 2006a ‡ 2002–2005 12.4 (88) 0 ± 51 -136 ± 19 -139 ± 73 + Ramillien et al., 2006 ‡ 2002–2005 14.1 (100) 67 ± 28 -107 ± 23 -129 ± 15 -169 ± 66 Wingham et al., 2006 * 1992–2003 8.5 (60) 27 ± 29 + Velicogna and Wahr, 2006b ‡ 2002–2006 1.7 (12) -227 ± 33 Chen et al., 2006 ‡ 2002–2005 1.7 (12) -219 ± 21 Luthcke et al., 2006 ‡ 2003–2005 1.7 (12) -101 ± 16 Stearns and Hamilton, 2007 # 2001–2006 1.0 (7) -122 ± 30 Rignot, 2008 † 1974–2007 N/A - 105 ± 27 Rignot et al., 2008 † 1992–2006 134.0 (N/A) - 4 ± 61 - 132 ± 60 - 60 ± 46 Range - 4 to 67 - 136 to –47 -139 to 42 -227 to 11

Notes: N/A – not available + Studies included in the IPCC AR4 report * Altimetry; † InSAR mass budget; ‡ Gravimetry; # Other method Source: Adapted from Table 1 in Shepherd and Wingham, 2007.

10 Environment Department Papers Recent Evidence for the Greenland and Antarctic Ice Sheets

Measurements by Howat et al. (2007) for two of retreat of ice in .17 Its neighboring Greenland’s largest outlet glaciers: Kangerdlugssuaq Thwaites Glacier is widening and may double its and Helheim located on the central east indicate width when the weakened eastern breaks up. a doubling of the rate of mass loss in less than a year in Widespread acceleration in this sector may be caused by 2004, and then a decrease in 2006 to near the previous glacier un-grounding from ice shelf melting by an ocean rates. Despite this wide year-over-year variation the that has recently warmed by 0.3°C. In contrast, glaciers overall trend in the velocity structure of the GIS is buffered from oceanic change by large ice shelves have clear – with a mass loss of about 100 Gt per year which only small contributions to sea level.18 is a considerable increase in the velocity structure from a near balance during the (Joughin et al., 2008; At present, many glaciers in East Antarctica are close Kohler et al., 2007; Luthcke et al., 2006; Thomas et al., to a state of mass balance, but sectors grounded well 2004; Van de Wal et al., 2008). below sea level, such as Cook Ice Shelf, Ninnis/Mertz, Frost and Totten glaciers, are thinning and losing mass. Hence, East Antarctica is not immune to changes. Antarctic Ice Sheet

Although low temperatures in Antarctica produce Vaughan (2006) has noted that long-term records from no surface runoff, recent satellite observations of ice meteorological stations on the Antarctic Peninsula show discharge have challenged the notion that the ice sheet strong rising trends in the annual duration of melting changes with eternal slowness. For example, using conditions. In each case, the trend is statistically measurements of time-variable gravity from the Gravity significant and represents a major increase in the Recovery and Climate Experiment (GRACE) , potential for melting. For example, between 1950 Velicogna and Wahr (2006a) determined that the and 2000 records from the Faraday/Vernadsky Station Antarctic ice sheet mass decreased significantly, at a rate indicated a 74% increase in the number of positive of 152 ± 80 km/yr of ice, equivalent to 0.4 ± 0.2 mm/ degree-days. A simple parameterization of the likely yr of global SLR, during the period 2002-2005, with effects of the warming on the rate of snow melt suggests most of this loss from the WAIS. The EAIS exhibits the an increase across the Antarctic Peninsula ice sheet smallest range of variability among recent mass balance from 28 ± 12 Gt/yr in 1950, to 54 ± 26 Gt/yr by 2000. estimates (Table 3), however losses from the WAIS Given a similar rate of warming over the next 50 years more than offset any growth occurring in the EAIS, this may reach 100 ± 46 Gt/yr, a figure comparable to leading to a net loss for the AIS as a whole. Thus most current loss estimates of the GIS. of the dynamic changes in the AIS are driven by losses in the WAIS. Recently, Rignot et al. (2008) used satellite interferometric synthetic-aperture radar observations Rignot (2006), with Advanced Land Observation covering 85% of Antarctica’s coastline to estimate the System Phased-array Synthetic-Aperture Radar (ALOS total mass flux into the ocean from 1992 to 2006. PALSAR) data, has pointed out that pronounced The mass fluxes from large drainage basin units were regional warming in the Antarctic Peninsula has compared with interior snow accumulation calculated triggered an ice shelf collapse, and in turn leading to from a regional atmospheric for a 10-fold increase in glacier flow and rapid ice sheet 1980 to 2004. According to these estimates, in East retreat. In West Antarctica, the Pine Island Bay sector Antarctica, small glacier losses in Wilkes Land and is draining far more ice into the ocean than is stored glacier gains at the mouths of the Filchner and Ross upstream from snow accumulation. This sector alone ice shelves combine to a near-zero loss of 4 ± 61 Gt/ could raise sea levels by 1m and trigger widespread yr; but in West Antarctica, widespread losses along the

Climate Change Series 11 Climate Change and Sea Level Rise — A Review of the Scientific Evidence

Bellingshausen and Amundsen increased the ice is significant, equivalent to (0.008–0.055) mm/ sheet loss by 59% in 10 years to reach 132 ± 60 Gt/yr yr of global sea level rise. Given future warming in 2006. In the Peninsula, losses increased by 140% to this could easily increase in the coming 50 years. reach 60 ± 46 Gt/yr in 2006. This contribution due to increased runoff could be augmented by any dynamic imbalance in the Although it is difficult to predict the fraction of glaciers draining the ice sheet. This finding appears to ablation that will become runoff, a calculation based contradict the conclusions of previous assessments, on an established criterion for runoff indicates that including the IPCC, which considered the contribution the contribution from the Antarctic Peninsula, as a of runoff from Antarctica to sea level rise would be direct and immediate response to climate warming insignificant.

12 Environment Department Papers

5 Other Projections of SLR

n light of the recent evidence on the vulnerability of (Zwally et al., 2002). Satellite images of the Greenland Greenland and West Antarctic Ice Sheets to climate ice sheet has revealed the appearance of large melt water warming, the conservative IPCC projections have pools on the ice surface in recent years (Moore, 2007). been criticized by a number of researchers, even In a study which combined satellite-derived estimates Iby other members within the IPCC (Solomon et al., of Arctic sea- and thickness to produce a 2008). Much of the debate surrounds the exclusion ice thickness record for 1982 to the present, revealed a of dynamical changes in the ice sheets which tend to much-reduced extent of the oldest and thickest ice19, produce much larger impact estimates. According leading to an enhanced risk of ice- feedback in to the IPCC, until sufficient research evidence Arctic (Maslanik et al., 2007).20 accumulates in this area, these uncertainties are too large to warrant inclusion in their predictions used for Even with these dynamic uncertainties recent satellite . observations point to accelerating trends in the rates of change among the contributors of SLR. Rahmstorf Hansen (2006, 2007) has been particularly vocal et al. (2007a) note that since 1990 the sea level has of the IPCC for its neglect of potential “ice sheet been rising faster than that projected by most climate disintegration” in projections of SLR, which have models. Satellite data show a linear trend in SLR remained relatively small until past few years. Due to of 3.3mm/year over the period 1993-2006, whereas the nonlinearity of the ice disintegration it is difficult to previous predictions by the IPCC (2001) projected a accurately predict the sea level change on a specific date, best estimate rise of less than 2mm/year. however the process is bounded by thresholds and once crossed, may trigger many meters of SLR well before An alternative to the IPCC AR4 SLR approach the end of this century. pioneered by Rahmstorf (2007b) estimates sea level rise indirectly from changes in global average The largest uncertainties are with respect to the multiple near surface temperature, one of the more accurate feedbacks occurring on and under the ice sheets and in variables in CGCMs. This semi-empirical technique the nearby oceans that accelerate the process of ice sheet estimates sea level rise based on changes in global disintegration. For example, a key feedback of the ice average temperature and sea level between 1880 and sheets is the ‘albedo flip’ that occurs when snow and the present. A proportionality constant of 3.4 mm/ ice begin to melt. Snow-covered ice reflects most of yr/ °C was computed from data on global SLR and the back to space however as warming causes global mean surface temperature for the 20th century. increased melting on the surface, the darker wet ice This proportionality constant applied to IPCC AR4 absorbs much more . Most of the resulting emission scenarios resulted in a projected SLR in 2100 melt water burrows through the ice sheet, lubricates its of 0.5 to 1.4 meters above the 1990 level. Using a base, and speeds the discharge of icebergs to the ocean wider range of CGCM models, Horton et al. (2008)

Climate Change Series 13 Climate Change and Sea Level Rise — A Review of the Scientific Evidence

update Rahmstorf’s results and produce a broader range shortage of observations characterizing how glaciers of sea level rise projections, especially at the higher end and ice sheets respond to changes in the climate, than outlined in the IPCC AR4. They predict sea level scientific inference can also be drawn from paleo- rise increases between 0.54 – 0.89m with a mean of climatic changes. Paleo-climatic information indicates 0.71m by 2100, however they admit that this does not that the warming of the last half-century is unusual in, include possible dynamic changes in the ice sheets. at least, the past 1300 years. The last time the Polar Regions were significantly warmer than the present Further support for the upper end of these results for an extended period of time was about 125,000 has been found by others using different techniques. years ago. Otto-Bliesner et al. (2007) used a global For example, Pfeffer et al. (2008) explore kinematic climate model, a dynamic ice sheet model, and paleo- scenarios of glacier contributions to sea level rise in climatic data to evaluate warming in the Northern the 21st century by setting 2m and 5m SLR targets Hemisphere and its impact on the Arctic ice fields and compare current loss rate evidence to see whether during the Last Interglaciation (LIG, ~116,000 – these objectives can be achieved. They find that a total 130,000 years ago). Their simulated climate matches SLR of about 2 meters by 2100 could occur under paleo-climatic observations of past warming, and the physically possible glaciological conditions but only if combination of physically based climate and ice-sheet all variables are quickly accelerated to extremely high modeling with ice-core constraints indicates that limits. More plausible but still accelerated conditions the Greenland Ice Sheet and other circum-Arctic ice lead to total sea-level rise by 2100 of about 0.8m. They fields likely contributed 2.2 to 3.4 meters of sea-level suggest that this estimate be a starting point for forecast rise during the LIG. Overpeck et al. (2006) using a refinements, but the range of 0.8 – 2.0m still remains a similar method conclude that under a business-as-usual possibility with the inclusion of ice flow dynamics. scenario, similar areas of Greenland could melt under the warmer environment, raising sea level by several In the absence of our complete understanding of meters by the end of the 21st century. the processes that control ice sheet behavior and the

14 Environment Department Papers

6 Conclusion

he IPCC AR4 report identifies several major 2008). Although there is continued evidence of ice factors that currently contribute to sea-level rise sheet growth in the Eastern regions of Antarctica (Davis (SLR): 1) ocean thermal expansion, 2) changes et al., 2005; Helsen et al., 2008; Ramillien et al., 2006; in glaciers and icecaps, 3) glacial melt from the Rignot and Thomas, 2002; Velicogna and Wahr, 2006a; TGreenland and Antarctica Ice Sheets, and 4) smaller Zwally et al., 2005), when coupled with the measured contributions from terrestrial storage, snow on land losses in the West, and including Greenland, the and permafrost. Anthropogenic warming and SLR evidence appears to point in the direction of increased will continue for centuries due to the time scales SLR. The implications of these recent observations associated with climate processes and feedbacks, even for sea-level rise could be dramatic: the Greenland if greenhouse gas concentrations were to be stabilized. and Antarctic ice sheets contain enough water to raise There appears to be a consensus across studies that the sea level by almost 70m, so even small changes in global sea level is projected to rise during the 21st their volume would have a significant effect. If the century at a greater rate than during the period 1961 to Greenland ice sheet were to melt completely, it would 2003 and unanimous agreement that SLR will not be raise average sea level by approximately 7 meters. geographically uniform.21 Ocean thermal expansion is projected to contribute significantly, and land ice will Paleo-climatic information also indicates that the increasingly lose mass at an accelerated rate. But most warmth of the last half-century is unusual in at-least the controversial are the mass balance loss estimates of the past 1300 years. The last time the Polar Regions were Greenland and Antarctic Ice Sheets and what the yet significantly warmer than at present for an extended un-quantified dynamic processes will imply in terms of period (about 125,000 years ago), reductions in polar SLR. ice volume led to 4 - 6 meters of sea level rise (IPCC, 2007). As new research of the impacts of warming The IPCC AR4 report predicted increased global SLR come in, we will be able to have more confidence in between 0.18m and 0.59m across various emission the ranges of impact, but current evidence leads to an scenarios over the next 100 years. However, this alarming possibility that a threshold triggering many range has been cast in doubt by many experts as meters of sea level rise could be crossed well before the being too conservative and not adequately reflecting end of this century (Hansen, 2006; Overpeck et al., uncertainty (Oppenheimer et al., 2008; Pfeffer et 2006). al., 2008; Solomon et al., 2008). New data on rates of deglaciation in Greenland and Antarctica suggest Impacts of SLR on storm surge height greater significance for glacial melt, especially due to the uncertainty surrounding the dynamics of outlet The implications of sea level rise are not only long-term glaciers (Helsen et al., 2008; Joughin et al., 2008; in . A commonly held belief is that since sea Kohler et al., 2007; Rignot, 2008; Van de Wall et al., level rise is rather slow and gradual, society has enough

Climate Change Series 15 Climate Change and Sea Level Rise — A Review of the Scientific Evidence

time to adapt to these new realities and thus immediate of model projections, the report states that it is likely alarm is not warranted. However even small changes in (greater than 66% probability) that future tropical sea level rise have a profound implication on the impact cyclones will become more intense, with larger peak of storm surges, which occur annually and typically wind speeds and heavier precipitation associated with with devastating consequences on coastal areas.22 Sea ongoing increases of tropical sea surface temperatures level rise basically acts as the baseline reference point (IPCC, 2007). A recent IWTC statement23 has also to which storm surge height is added. If the baseline asserted that “…if the projected rise in sea level due rises at a faster rate than what was originally believed to global warming occurs, then the vulnerability to (or invested upon), storm surges become even more tropical cyclone storm surge flooding would increase” unpredictable in their damage capability. and “…it is likely that some increase in tropical cyclone peak wind-speed and rainfall will occur if the climate continues to warm. Model studies and project The IPCC AR4 suggested that, globally, estimates of a 3-5% increase in wind-speed per degree Celsius the potential destructiveness of hurricanes/ cyclones increase of tropical .” Given show a significant upward trend since the mid-1970s, the consensus of an ever increasing temperature, the with a trend towards longer lifetimes and greater storm severity of storm surges is one aspect of climate change intensity, and such trends are strongly correlated with that countries will have to contend with even in the tropical sea surface temperature. Based on a range short-term.

16 Environment Department Papers

References

Bahr, D. B., M. Dyurgerov, and M. F. Meier, 2009: Cogley, J.G., 2005: Mass and energy balances of Sea-level rise from glaciers and ice caps: A lower bound, glaciers and ice sheets. In: Encyclopedia of Hydrological Geophysical Research Letters 36: L03501. [Anderson, M. (ed.)]. John Wiley & Sons, Ltd, Chichester, pp 2555–2573. Bamber, J., R. Layberry and S.Gogineni, 2001: A new ice thickness and bed data set for the Greenland Cubasch, U., B.D. Santer, A. Hellbach, G. Hegerl, H. ice sheet, 1. Measurement, datareduction, and errors Höck, E. Meier- Reimer, U. Mikolajewicz, A. Stössel, Journal of Geophysical Research 106: 33733–33780. R. Voss, 1994: Monte Carlo climate change forecasts with a global coupled ocean-atmosphere model Climate Braithwaite, R.J., Y. Zhang, and S.C.B. Raper, Dynamics 10: 1-19. 2003: Temperature sensitivity of the mass balance of mountain glaciers and ice caps as a climatological Dasgupta, S., B. Laplante, C. Meisner, D. Wheeler, characteristic Zeitschrift für Gletscherkunde und J. Yan (2007). “The Impact of Sea-Level Rise on Glazialgeologie 38: 35–61. Developing Countries: A Comparative Analysis”. World Bank Policy Research Working Paper # 4136. Bryan, K., 1996: The steric component of sea level rise associated with enhanced greenhouse warming: a model Davis, C. and Y. Li, paper presented at for study Climate Dynamics 12: 545-55. Society: Exploring and Managing a Changing Planet (IEEE, Anchorage, Alaska, 20–24 Sep 2004), pp. Chen, J., C. Wilson, B. Tapley, 2006: Satellite 1152–1155. gravity measurements confirm accelerated melting of Greenland Ice Sheet Science 313: 1958-1960. Davis, C., Y. Li, J. McConnell, M. Frey, E. Hanna, 2005: Snowfall-driven growth in East Antarctic Ice Church J, Gregory J, Huybrechts P, Kuhn M, Lambeck Sheet mitigates recent sea-level rise Science 308: 1898- K, Nhuan M, Qin D, Woodworth P (2001) Changes 1901. in sea level. In: Houghton J, Ding Y, Griggs D, Noguer M, van der Linden P, Xiaosu D (eds.) Climate Change Dyurgerov, M., and M.F. Meier, 2005: Glaciers and the 2001. The Scientific Basis. Cambridge University Press, Changing Earth System: A 2004 Snapshot. Occasional Cambridge, pp. 639–693. Paper 58, Institute of Arctic and Alpine Research, University of , Boulder, CO, 118 pp. Chylek, P., and U. Lohmann, 2005: Ratio of the Greenland to global temperature change: Gregory, J.M., 1993: Sea level changes under increasing Comparison of observations and climate modeling atmospheric CO2 in a transient coupled ocean- results. Geophysical Research Letters 32: L14705, atmosphere GCM Experiment Journal of Climate 6: doi:10.1029/2005GL023552. 2247-2262.

Climate Change Series 17 Climate Change and Sea Level Rise — A Review of the Scientific Evidence

Gregory, J.M. and J.A. Lowe, 2000: Predictions of Horton, R., C. Herweijer, C. Rosenzweig, J. Liu, global and regional sea level rise using AOGCMs with V. Gornitz, and A. C. Ruane, 2008: Sea level rise and without flux adjustment Geophysical Research projections for current generation CGCMs based Letters 27: 3069-3072. on the semi-empirical method Geophysical Research Letters 35: L02715. Gregory, J.M., P. Huybrechts, and S.C.B. Raper, 2004: Threatened loss of the Greenland ice-sheet Nature 428: Howat I., I. Joughin, T. Scambos, 2007: Rapid changes 616. in ice discharge from Greenland outlet glaciers Science 315: 1559-1561. Gregory, J.M., and P. Huybrechts, 2006: Ice-sheet contributions to future sea-level change Philosophical Huybrechts, P., J. Gregory, I. Janssens, and M. Wild, Transactions of the Royal Society Series A 364: 2004: Modelling Antarctic and Greenland volume 1709–1731. changes during the 20th and 21st centuries forced by GCM time slice integrations Global Planetary Change Hanna, E.,, P. Huybrechts, I. Janssens, J. Cappelen, K. 42: 83–105. Steffen & A. Stephens, 2005: Runoff and mass balance of the Greenland Ice Sheet: 1958-2003 Journal of IPCC, 2001: Climate Change 2001: The Scientific Geophysical Research 110: D13108. Basis – Contribution of Working Group I to the IPCC Third Assessment Report 2001. Hansen, J., 2007: Scientific reticence and sea level rise Environmental Research Letters 2. IPCC, 2007: Climate Change 2007: The Physical Science Basis, Summary for Policymakers. Hansen, J., M. Sato, P. Kharecha, G. Russell, D.W. Lea, and M. Siddall, 2007: Climate change and trace gases International Workshop on Tropical Cyclone (IWTC), Philosophical Transactions of the Royal Society Series A 2006: Statement on tropical cyclones and climate 365: 1925-1954. change. November, 2006, 13 pp.

Hansen, J., L. Nazarenko, R. Ruedy, M. Sato, J. Willis, Available at: http://www.gfdl.noaa.gov/~tk/glob_warm_ A. Del Genio, D. Koch, A. Lacis, K. Lo, S. Menon, T. hurr.html Novakov, J. Perlwitz, G. Russell, G.A. Schmidt, and N. Tausnev, 2005: Earth’s energy imbalance: confirmation Jackett, D.R., T.J. McDougall, M.H. England, and and implications Science 308: 1431–1435. A.C. Hirst, 2000: Thermal expansion in ocean and coupled general circulation models Journal of Climate Hansen, J., 2006: Can we still avoid dangerous human- 13: 1384-1405. made climate change? Presentation on December 6, 2005 to the American Geophysical Union in Joughin, I, S. Das, M. King, B. Smith, I. Howat, T. San Francisco, California. Available at: http://www. , 2008: Seasonal speedup along the western flank columbia.edu/~jeh1/newschool_text_and_slides.pdf. of the Greenland ice sheet Science 320: 781-783.

Helsen, M., M. van den Broeke, R. van de Wal, W. Krabill, W., W. Abdalati, E. Frederick, S. Manizade, C. van de Berg, E. van Meijgaard, C. Davis, Y. Li, I. Martin, J. Sonntag, R. Swift, R. Thomas, W. Wright, Goodwin, 2008: Elevation changes in Antarctica J. Yungel, 2000: Greenland Ice Sheet: High-elevation mainly determined by accumulation variability Science balance and peripheral thinning Science 289: 428-430. 320: 1626-1629.

18 Environment Department Papers References

Simulating Arctic Climate Warmth and Icefield Retreat Rignot, E. and R. Thomas, 2002: Mass balance of polar in the Last Interglaciation Science 311:1751-1753. ice sheets Science 297: 1502-1506.

Overpeck, J., B. Otto-Bliesner, G. Miller, D. Muhs, R. Rignot, E. and P. Kanagaratnam, 2006: Changes in the Alley, J. Kiehl, 2006: Paleoclimatic evidence for future velocity structure of the Greenland Ice Sheet Science ice-sheet instability and rapid sea-level rise Science 311: 311: 986–990. 1747-1750. Robinson, D., K. Dewey and R. Heim Jr., 1993: Pfeffer, W. T., J. T. Harper, S. O’Neel, 2008: Kinematic Global snow cover monitoring: an update Bulletin of constraints on glacier contributions to 21st-century sea- the American Meteorological Society 74: 1689–1696. level rise Science 321: 1340-1343. Shepherd, A. and D. Wingham, 2007: Recent sea-level Rahmsdorf, S., A. Cazenave, J. Church, J. Hansen, contributions of the Antarctic and Greenland ice sheets R. Keeling, D. Parker, R. Somerville, 2007a: Recent Science 315: 1529-1532. climate observations compared to projections Science 316: 709. Solomon, S., R. Alley, J. Gregory, P. Lemke, M. Manning, 2008: A closer look at the IPCC Report Rahmsdorf, S., 2007b: A semi-empirical approach to Science 319: 409-410. projecting future sea-level rise Science 308: 368-370. Stearns, L and G. Hamilton, 2007: Rapid volume loss Ramillien, G., A. Lombard, A. Cazenave, E. Ivins, from two East Greenland outlet glaciers quantified M. Llubes, F. Remy, R. Biancale, 2006: Interannual using repeat stereo Geophysical variations of the mass balance of the Antarctica Research Letters 34: L05503. and Greenland ice sheets from GRACE Global and Planetary Change 53: 198-208. Stroeve J, M. Holland, W. Meier, T. Scambos, M. Serreze, 2007: Arctic decline: Faster than forecast Ridley, J.K., P. Huybrechts, J.M. Gregory, and J.A. Geophysical Research Letters 34: L09501. Lowe, 2005: Elimination of the Greenland ice sheet in a high CO2 climate Journal of Climate 17: 3409-3427. Tedesco, M. 2007: Snowmelt detection over the Greenland ice sheet from SSM/I brightness temperature Rignot, E., J.L. Bamber, M.R. van den Broeke, daily variations Geophysical Research Letters 34: C. Davis, Y. Li, W. J. van de Berg and E. van L02504. Meijgaard, 2008: Recent Antarctic ice mass loss from radar interferometry and regional climate modelling. Thomas, R., E. Rignot, G. Casassa, P. Kanagaratnam, Nature Geoscience 1: 106-110. C. Acuña, T. Akins, H. Brecher, E. Frederick, P. Gogineni, W. Krabill, S. Manizade, H. Ramamoorthy, Rignot, E., 2008: Changes in West Antarctic ice A. Rivera, R. Russell, J. Sonntag, R. Swift, J. Yungel, stream dynamics observed with ALOS PALSAR data J. Zwally, 2004: Accelerated sea-level rise from West Geophysical Research Letters 35: L12505. Antarctica Science 306: 255-258.

Rignot, E. 2006: Changes in ice dynamics and mass Van de Wal, R. S., W. Boot, M. van den Broeke, C. balance of the Antarctic ice sheet Philosophical Smeets, C. Reijmer, J. Donker, J. Oerlemans, 2008: Transactions of the Royal Society Series A 364: 1637- Large and rapid melt-induced velocity changes in the 1655.

Climate Change Series 19 Climate Change and Sea Level Rise — A Review of the Scientific Evidence

Krabill, W, E. Hanna, P. Huybrechts, W. Abdalati, J. Moore, F., 2007: Putting climate change in a geologic Cappelen, B. Csatho, E. Frederick, S. Manizade, C. context - are models under-predicting future changes in Martin, J. Sonntag, R. Swift, R. Thomas, J. Yunge, sea level? The Climate Institute. 2004: Greenland ice sheet: increased coastal thinning Geophysical Research Letters 31: L24402. Available at: http://www.climate.org/topics/sea-level/ climate-change-sea-level-rise.html Kohler, J., T. James, T. Murray, C. Nuth, O. Brandt, N. Barrand, H. Aas, and A. Luckman, 2007: Acceleration Nicholls, R., 2003: An expert assessment of storm in thinning rate on western glaciers surge “hotspots”. Final report (draft version) for the Geophysical Research Letters 34: L18502. Center for and Risk Research, Lamont-Dohert Observatory, Columbia University. Leuliette, E. W., and L. Miller, 2009: Closing the sea level rise budget with altimetry, , and GRACE, Oerlemans, J., B. Anderson, A. Hubbard, P. Geophysical Research Letters 36: L04608. Huybrechts, T. Jóhannesson, W. Knap, M. Schmeits, A. Stroeven, R. van de Wal, J. Wallinga, Z. Zuo, 1998: Luthcke, S.B., H.J. Zwally, W. Abdalati, D.D. Modeling the response of glaciers to climate warming Rowlands, R.D. Ray, R.S. Nerem, F.G. Lemoine, Climate Dynamics 14: 267–274. J.J. McCarthy, D.S. Chinn, 2006: Recent Greenland ice mass loss by drainage system for satellite gravity Oerlemans, J., 2001: Glaciers and Climate Change. A. observations Science 314: 1286-1289. A. Balkema, Lisse, The , 148 pp.

Lythe, M.B., D.G. Vaughan, and the BEDMAP Group, Oerlemans, J., R. Bassford, W. Chapman, J. 2001: BEDMAP: A new ice thickness and subglacial Dowdeswell, A. Glazovsky, J.-O. Hagen, K. Melvold, topographic model of Antarctica Journal of Geophysical M. de Ruyter de Wilde, R. van de Wal, 2006: Research 106: 11335–11351. Estimating the contribution from Arctic glaciers to sea-level change in the next hundred years Annals of Maslanik, J. A., C. Fowler, J. Stroeve, S. Drobot, J. Glaciology 42: 230–236. Zwally, D. Yi, and W. Emery, 2007: A younger, thinner Arctic ice cover: Increased potential for rapid, extensive Ohmura, A., 2004: during the twentieth sea-ice loss Geophysical Research Letters 34: L24501. century. In: The State of the Planet: Frontiers and Challenges in [Sparks, R.S.J. and C.J. McLean, R., and A. Tsyban, 2001: Coastal zone and Hawkesworth (eds.)]. Geophysical Monograph 150, marine . In: McCarthy, J.J., Canziani, International Union of Geodesy and Geophysics, O.F., Leary, N.A., Dokken, D.J., and White, K.S. Boulder, CO and American Geophysical Union, (eds.) Climate Change 2001: Impacts, Adaptation and Washington, DC, pp. 239–257. Vulnerability, Cambridge University Press, Cambridge, pp. 343–380. Oppenheimer, M., B. O’Neill, M. Webster, S. Agrawala, 2007: The limits of consensus Science 317: Meier M, M. Dyurgerov, U. Rick, S. O’Neel, W. 1505-1506. Pfeffer, R. Anderson, S. Anderson, A. Glazovsky, 2007: Glaciers dominate eustatic sea-level rise in the 21st Otto-Bliesner, B., S. Marshall, J. Overpeck, G. Miller, century Science 317: 1064-1067. A. Hu, CAPE Last Project members, 2006:

20 Environment Department Papers References

ablation zone of the Greenland ice sheet Science 321: Zhang, T., R. Barry, K. Knowles, J. Heginbottom, 111-113. J. Brown, 1999: Statistics and characteristics of Permafrost and ground ice Distribution in the Vaughan, D.J., 2006: Recent trends in melting Northern Hemisphere Polar Geography 23:132-154. conditions on the Antarctic Peninsula and their implications for ice-sheet mass balance and sea level Zhang, T., et al., 2003: Distribution of seasonally and Arctic, Antarctic and Alpine Research 38(1): 147-152. perennially frozen ground in the Northern Hemisphere. In: Proceedings of the 8th International Conference Velicogna, I. and J. Wahr, 2005: Greenland mass on Permafrost, 21-25 July 2003, Zurich, balance from GRACE Geophysical Research Letters 32: [Phillips, M., S.M. Springman, and L.U. Arenson L18505. (eds.)]. A.A. Balkema, Lisse, the Netherlands, pp. 1289–1294. Velicogna, I. and J. Wahr, 2006a: Acceleration of Greenland ice mass loss in spring 2004 Nature 443: Zuo, Z., and J. Oerlemans, 1997: Contribution of 329-331. glacial melt to sea level rise since AD 1865: A regionally differentiated calculation Climate Dynamics 13: Velicogna, I. and J. Wahr, 2006b: Measurements of 835–845. time-variable gravity show mass loss in Antarctica Science 311: 1754–1756. Zwally, H., W. Abdalati, T. Herring, K. Larson, J. Saba and K. Steffen K, 2002: Surface melt-induced Wingham, D., A. Ridout, R. Scharroo, R. Arthern, C. acceleration of Greenland ice-sheet flow Science 297: Shum, 1998: Antarctic elevation change from 1992 to 218–222. 1996 Science 282: 456-458. Zwally, H., M. Giovinetto, J. Li, H. Cornejo, M. Wingham, D., A. Shepherd, A. Muir, G. Marshall, Beckley, A. Brenner, J. Saba, D. Yi, 2005: Mass changes 2006: Mass balance of the Antarctic ice sheet of the Greenland and Antarctic ice sheets and shelves Philosophical Transactions of the Royal Society Series A and contributions to sea-level rise: 1992-2002 Journal 364: 1627-1635. of Glaciology 51: 509-527.

Climate Change Series 21 Appendix 1 — Source of Data on Major Contributory Factors to SLR

Ocean Thermal Expansion core sites using satellite microwave measurements or radar sounding. Increasingly, atmospheric stimates of change in global mean temperature modeling techniques are also applied. and sea level rise due to thermal expansion • Ice discharge is calculated from radar or seismic are usually made using climate models; and measurements of ice thickness, and from in situ or AOGCMs have been shown to be the most remote measurements of ice velocity, usually where Esatisfactory way of estimating ocean thermal expansion the ice begins to float and velocity is nearly depth- (Gregory, 1993; Cubasch et al., 1994; Bryan, 1996; independent. A major advance in recent years has Jackett et al., 2000; Russell et al., 2000; Gregory and been widespread application of Interferometric Lowe, 2000). The advantage of using such models is Synthetic Aperture Radar (InSAR) techniques from the models include dynamical components describing satellites to measure ice velocity over very large ar- atmospheric, oceanic and land surface processes, as well eas of the ice sheets. Calculation of mass discharge as sea ice and other components. There are now over 20 also requires estimates for runoff of surface melt AOGCMs from different centers available for climate water. Surface melt amounts usually are estimated simulations.24,25 from modeling driven by atmospheric re-analyses, global models or , and often calibrated Greenland Ice Sheet, Antarctic Ice Sheet against surface observations where available. • However, ice sheet mass inputs and outputs are Standard techniques used to measure the mass balance difficult to estimate with high accuracy. Although of large ice masses are as follows: broad InSAR coverage and progressively improving estimates of grounding-line ice thickness have • The mass budget approach compares input from substantially improved ice discharge estimates, snow accumulation with output from ice flow and incomplete data coverage implies uncertainties in melt water runoff discharge estimates of a few percent. Glacier veloci- • Repeated altimetry measures surface elevation ties also can change substantially, sometimes in changes months or years, adding to the overall uncertainty • Temporal variations in gravity over the ice sheets of mass budget calculations. reveal mass changes • Changes in day length and in the direction of the Repeated altimetry measures surface elevation changes: Earth’s rotation axis also reveal mass redistribution. • Surface elevation changes reveal ice sheet mass The mass budget approach: changes after correction for changes in depth- density profiles and in bedrock elevation, or for • Snow accumulation is often estimated from annual hydrostatic equilibrium if the ice is floating. layering in ice cores, with interpolation between

Climate Change Series 22 Appendix 1 — Source of Data on Major Contributory Factors to SLR

Satellite radar altimetry (SRALT) has been widely directly measured glacier mass balances are few and used to estimate elevation changes together with stretch back only to the mid-20th century. Because laser altimetry from airplanes and from the Ice, of the very intensive fieldwork required, these records and land Elevation Satellite (ICESat). Mod- are biased towards logistically and morphologically eled corrections for isostatic changes in bedrock ‘easy’ glaciers. Uncertainty in directly measured annual elevation are small (a few millimeters per year), but mass balance is typically ± 200 kg / m2 /yr due to with uncertainties nearly as large as the corrections measurement and analysis errors (Cogley, 2005). in some cases. Corrections for near-surface firn density changes are larger and also uncertain. Mass balance data are archived and distributed by the • Radar altimetry has provided long-term and World Glacier Monitoring Service (WGMS(ICSI- widespread coverage for more than a decade. IAHS), various years-b). From these and from several other new and historical sources, quality checked Temporal variations in gravity over the ice sheets reveal time series of the annual mean specific mass balance mass changes: (the total mass balance of a glacier or ice cap divided by its total surface area) for about 300 individual • Since 2002, the GRACE satellite mission has been glaciers have been constructed, analyzed and presented providing routine measurement of the Earth’s in three databases (Ohmura, 2004; Cogley, 2005; gravity field and its temporal variability. After Dyurgerov and Meier, 2005). Recent findings from removing the effects of , atmospheric load- repeat altimetry of glaciers and ice caps in Alaska ing, etc., high-latitude data contain information and Patagonia have also been incorporated in these on temporal changes in the mass distribution of databases. the ice sheets and underlying rock. Estimates of ice sheet mass balance, however, are sensitive to modeled estimates of bedrock vertical motion, Snow on land primarily arising from response to changes in mass The premier data set used to evaluate large-scale snow loading from the end of the last ice age.) covered area dates to 1966, and is the weekly visible wavelength satellite maps of northern hemisphere Other Methodologies: snow cover produced by the US National Oceanic • Data on changing length of day from eclipse re- and Atmospheric Administration’s (NOAA) National cords, the related ongoing changes in the spherical- Environmental Satellite Data and Information harmonic coefficients of the geo-potential, and Service (NESDIS; Robinson et al., 1993). Trained true polar wander also reveal mass redistribution. meteorologists produce the weekly NESDIS snow At present, unique solutions are not possible from product from visual analyses of visible satellite these techniques, but hypothesized histories of ice imagery. These maps are well validated against surface sheet changes can be tested against the data for observations, although changes in mapping procedures consistency, and is rapidly being made. in 1999 affected the continuity of data series at a small number of mountain and coastal grid points. For Glaciers and icecaps (not immediately the southern hemisphere, mapping of snow covered adjacent to Greenland and Antarctic ice area began only in 2000 with the advent of Moderate sheets) Resolution Imaging Spectroradiometer (MODIS) satellite data. Although written reports (and records) of glacier length changes go far back as 1600 in a few cases; records of

Climate Change Series 23 Climate Change and Sea Level Rise — A Review of the Scientific Evidence

Since 1978, space-borne passive microwave sensors Permafrost offer the potential for global monitoring of not just The source of information on permafrost temperature snow cover, but also snow depth and “Snow Water is monitored data. Systematic permafrost temperature Equivalent”, unimpeded by and monitoring started in northern Alaska in the 1940s, in darkness. Russia in the 1950s, in in the early 1980s, in the Tibetan Plateau in the late 1980s, and in Europe in the 1990s.

24 Environment Department Papers Appendix 2 — The Emission Scenarios of the IPCC Special Report

n 1992 the IPCC released emissions scenarios to scenario family describes a more heterogeneous be used for driving global circulation models to world, where the underlying theme is self-reliance and develop climate change scenarios. The so-called preservation of local identities. Convergence in fertility IS92 scenarios were the first global scenarios to rates occurs very slowly across regions leading to an Iprovide estimates for the full suite of greenhouse increasing global population. Economic development gases. Acknowledging the limitations of the first set is regionally oriented and per capita of scenarios, the IPCC published a Special Report on and technological change are more fragmented and Emissions Scenarios (SRES) in 2000 (IPCC, 2000) slower than in other storylines. that form the basis of analyzing the impact of human demographic, economic, political and technological The second group and family scenarios (B1) create changes on future emissions. The SRES report a very different world from the A-series in terms of supersedes the IS92 emission scenarios. To describe overall emissions intensity. This scenario describes a alternative activities the report consists of four major convergent world with the same global population ‘storylines’ or scenarios that develop quantitative that peaks in mid-century and declines thereafter, estimates of the socio-economic drivers of greenhouse as in the A1 storyline, but with rapid changes in and emissions, including factors such as economic structures toward a service and information population, GDP and technology and in turn emission economy, with reductions in material intensity, scenarios. The purpose of these outputs is to provide and the introduction of clean and resource-efficient a consistent input to both climate models and impact technologies. The emphasis is on global solutions to assessment models. To avoid any potential bias, the economic, social, and environmental , IPCC states that each group of scenarios is equally including improved equity, but without additional sound and thus do not attach any probability (or climate initiatives. The last group, B2, describes a likelihood) to any one scenario. world in which the emphasis is on local solutions to The first group or family of scenarios (A1) feature a economic, social, and environmental sustainability. It world with rapid economic growth, global population possesses continuously increasing global population that peaks in mid-century and declines thereafter, at a rate greater than A2, intermediate levels of and the rapid introduction of new and more economic development, and less rapid and more efficient technologies. Major underlying themes diverse technological change than in B1 and A1 are convergence between regions, capacity building, storylines. While the scenario is also oriented toward and increased cultural and social interactions, with and social equity, it focuses a substantial reduction in regional differences in per on local and regional levels. capita income. A1 is subdivided into A1FI (fossil- intensive), A1T (high-technology), and A1B Each storyline was fed into an integrated assessment (balanced), with A1FI generating the most CO2 climate model to predict future and A1T the least. The A2 storyline and emissions26 and their ambient concentrations.

Climate Change Series 25

Endnotes

1. Satellite measurements since 1993 provide equivalent (SLE) of sea ice, ice shelves and unambiguous evidence of regional variability of seasonally frozen ground is approximately zero sea level change. Sea level changes do not occur (Lythe et al., 2001; Zhang et al., 2003). uniformly across all areas of the world. The largest SLR since 1992 has taken place in the Western 5. Approximately 360 Gt of ice = 1 mm of sea level Pacific and Eastern Indian Oceans. Nearly all of equivalent. the shows SLR during the past decade, while sea levels in the Eastern Pacific and 6. Smaller contributors have been omitted. Western Indian Oceans are falling (IPCC 2007). 7. This difference indicates a deficiency in current 2. Despite its conservative projections, the IPCC scientific understanding of sea level change and (2007) asserted that, “Paleo-climate information may imply an underestimate in projections supports that the warmth of the last half century (IPCC, 2007). is unusual in at-least the previous 1300 years. The last time the polar regions were significantly 8. A recent analysis of the steric and ocean mass warmer than present for an extended period components of sea level, however show that (about 125,000 years ago), reductions in polar ice the sea level rise budget for the period January volume led to 4-6 meters of sea level rise.” 2004 to December 2007 can be closed. Using corrected and verified Jason-1 and Envisat 3. As seawater warms up, it expands, increasing the altimetry observations of total sea level, upper volume of the global sea level and thermosteric ocean steric sea level from the Argo array, and SLR. ocean mass variations inferred from GRACE gravity mission observations, it has been found 4. The contribution of terrestrial storage (e.g., water that the sum of steric sea level and the ocean mass stored in the ground, lakes and reservoirs) is component has a trend of 1.5 ± 1.0 mm/a over difficult to estimate with confidence as systematic the period, which is in agreement with the total information is not available for various regions sea level rise observed by either Jason-1 (2.4 ± 1.1 of the world, and the interactions between SLR, mm/a) or Envisat (2.7 ± 1.5 mm/a) within a 95% aquifer withdrawals, lakes and reservoirs is mostly confidence interval (Leuliette and Miller, 2009). unknown. Thus terrestrial storage was omitted (IPCC, 2007). It is also important to note that 9. See Appendix A for an explanation of methods. changes in sea ice do not directly contribute to sea level change since the ice is already floating 10. Climate models are an imperfect representation and displacing water, although it can contribute of the earth’s climate system and climate modelers to salinity changes through the input of fresh employ a technique called ensembling to capture water. Changes in the mass of ice shelves, which the range of possible climate states. A climate are already floating, can affect the flow of adjacent model run ensemble consists of two or more ice that is not floating, and thus can affect SLR climate model runs made with the exact same indirectly (IPCC, 2007). Hence the sea level climate model, using the exact same boundary

26 Environment Department Papers Endnotes

forcings, where the only difference between 17. The Pine Island Glacier loss accelerated 38% the runs is the initial conditions. An individual since 1975, and most of this occurred over the simulation within a climate model run ensemble last decade. More recently this has increased to is referred to as an ensemble member. The 42% and has become ungrounded over most different initial conditions result in different of its ice plain from 1996 to 2007. The Smith simulations for each of the ensemble members due glacier loss has also accelerated by 83% and is now to the nonlinearity of the climate model system. ungrounded as well. Essentially, the earth’s climate can be considered to be a special ensemble that consists of only 18. European Space Agency’s ENVISAT images one member. Averaging over a multi-member have detected new rifts on the Wilkins ice shelf ensemble of model climate runs gives a measure in November, 2008. The Wilkins ice shelf has of the average model response to the forcings been stable for most of the last century and imposed on the model. began retreating in the 1990s. In February 2008, an area of about 400 km2 broke off, between 11. In particular, many of the AOGCM experiments May and July, the ice shelf experienced further do not include the influence of Mt. Pinatubo, disintegration and lost 1,350 km2. The new rifts the omission of which may reduce the projected on the ice shelf could lead to the opening of an rate of thermal expansion during the early 21st ice bridge which has been preventing the ice shelf century. from fully disintegrating and breaking away from the Antarctic peninsula (http://www.sciencedaily. 12. Ablation is the surface removal of ice or snow from com/releases/2008/11/081128132029.htm ). a glacier or snowfield by melting, sublimation, If the ice shelf actually breaks away from the and/or calving (ice sheering off to form icebergs). peninsula, it will not cause a rise in sea level since it is already floating, however it may allow ice 13. As glacier volume is lost, glacier area declines so sheets on land to move faster and adding new the ablation decreases. water to the seas.

14. A recent study of snow accumulation areas of 19. The oldest ice types have essentially disappeared, glaciers and icecaps has concluded that, at present, and 58% of multiyear ice now consists of relatively observed accumulation areas are small, forcing young 2- and 3-year-old ice compared to 35% in glaciers to lose 27% of their volume to attain the mid-1980s. equilibrium with current climate. As a result, at least 184 ± 33 mm of sea-level rise are forced by 20. Arctic ice coverage in the summer of 2007 reached mass wastage of the world’s mountain glaciers a record minimum, with ice extent declining by and ice caps even if the climate does not continue 42% compared to conditions in the 1980s. The to warm. If the climate continues to warm along much-reduced extent of the oldest and thickest current trends, a minimum of 373 ± 21 mm of ice, in combination with other factors such as ice sea-level rise over the next 100 years is expected that assist the ice-albedo feedback by from glaciers and ice caps (Bahr et al., 2009). exposing more open water, help explain this large and abrupt ice loss. 15. Antarctic mass variability is difficult to measure because of the ice sheet’s size and complexity. 21. The emphasis on demonstrating a consensus in Standard estimates use a variety of techniques, IPCC reports has put the spotlight on expected each with intrinsic limitations and uncertainties. outcomes, generating a reliance on exact numbers in the minds of policy-makers. With 16. IPCC AR3 suggested a SLR of 0.09 to 0.88 m by the general credibility of the science of climate the year 2100 unless greenhouse gas emissions are change established, it is now equally important reduced substantially (IPCC, 2001). that policy-makers understand the more extreme

Climate Change Series 27 Climate Change and Sea Level Rise — A Review of the Scientific Evidence

possibilities that consensus may exclude or 25. Estimates of change in global mean temperature downplay (Oppenheimer et al., 2008). and sea level rise due to thermal expansion can also be made with Simple Climate Models 22. In the last 200 years, at least 2.6 million (SCMs) and Earth System Models of Intermediate people may have drowned due to storm surges Complexity (EMICs). SCMs represent the and causing a range of other damages and ocean-atmosphere system as a set of global or disruptions (Nicholls, 2003). The tropical hemispheric boxes, and predict global surface cyclone Sidr in Bangladesh in November 2007 temperature using an energy balance equation, a and cyclone Nargis in the Irrawady Delta of prescribed value of and a basic in May 2008 are recent reminders representation of ocean heat uptake. Such models of the potentially devastating impacts of severe can also be coupled to simplified models of events and storm surges in less developed biogeochemical cycles and allow rapid estimation countries. According to the Bangladesh of the climate response to a wide range of emission Management Information Centre (report dated scenarios. EMICs, on the other hand, include Nov 26, 2007) 3,243 people were reported some dynamics of the atmospheric and oceanic to have died and the livelihoods of 7 million circulations, or parametrizations thereof and people were affected by the tropical cyclone often include representations of biogeochemical Sidr (http://www.reliefweb.int/rw/RWB.NSF/ cycles, but they commonly have reduced spatial db900SID/EDIS-79BQ9Z?OpenDocument). resolution. These models can be used to investigate In Mayanmar, 100,000 people were reported to continental-scale climate change and long-term, have died and 1.5 million people were affected by large-scale effects of coupling between Nargis (http://www.dartmouth.edu/%7Efloods/ system components using large ensembles of Archives/2008sum.htm). model runs or runs over many centuries.

23. International Workshop on Tropical Cyclones 26. Included are anthropogenic emissions of carbon (IWTC). dioxide (CO2), (CH4), (N2O), (HFCs), 24. Although the large-scale dynamics of these models perfluorocarbons (PFCs), sulfur hexafluoride are comprehensive, parameterizations are still used (SF6), hydrochlorofluorocarbons (HCFCs), to represent unresolved physical processes such as (CFCs), the aerosol precursor the formation of and precipitation, ocean and the chemically active gases mixing due to wave processes and the formation of (SO2), (CO), nitrogen oxides water masses, etc. Uncertainty in parameterization (NOx), and non-methane volatile organic is the primary reason why climate projections compounds (NMVOCs). differ between different AOGCMs.

28 Environment Department Papers Paper number 118 ENVIRONMENT DEPARTMENT PAPERS Climate Change Series Environment Department THE WORLD BANK

1818 H Street, NW Washington, D.C. 20433 Telephone: 202-473-3641 Facsimile: 202-477-0565 Climate Change and Sea Level Rise A Review of the Scientific Evidence

Susmita Dasgupta and Craig Meisner

May 2009

Sustainable Development Vice Presidency Printed on recycled paper stock, using soy inks.